The present invention relates to a coated cutting tool, suitable for chip forming machining of metals, and a method for producing the same. According to the present invention, there is provided a reliable method for removing coating layers on selected faces of a cutting insert during coating post-treatment.
Modern high productivity chip forming machining of metals requires reliable tools with excellent wear properties. This is achieved by employing a cemented carbide tool body coated with a wear resistant coating, of single layer or multilayer type, most commonly comprising wear layers of TiC, TiN, TiCN and Al2O3. For depositing the different layers onto the cemented carbide body, CVD, PVD, or similar coating techniques are used.
EP-A-693574 describes how different parts of a tool are subject to different types of wear during a machining operation. Since the various coating layers have different abilities to withstand the different types of wear, it is suggested to have an outermost Al2O3 layer on the rake face, because of its ability to withstand diffusion type wear, and on the clearance side it is suggested to have an outermost MeCxNyOz type layer, where Me is a metal selected from groups IVB, VB, VIB of the periodic table, because of its high resistance to flank wear. A top layer of TiCxNyOz or, in particular, a goldish TiN, ZrN or HfN top layer also makes it easy to differentiate between a used and an unused cutting edge by the naked eye. Hence, the TiCxNyOz layer is mechanically removed from either only the edge line or from both the rake face and the edge line to expose the Al2O3 layer. Normally this is done by a post-treatment such as blasting or brushing of the coated inserts.
During the post-treatment, it is important not to reduce the Al2O3 layer thickness along the edge line. The method must therefore be so gentle that only the top TiCxNyOz layer is removed, leaving the Al2O3 at the edge line as untouched as possible. However, the described post-treatment method is unreliable as residues of TiCxNyOz occasionally appear on the Al2O3 surface after blasting process. TiCxNyOz residues on the Al2O3 surface reduce the flaking resistance, due to welding of TiCxNyOz to the work piece at the cutting edge resulting in coating withdrawal and a lower lifetime of the insert. A second effect of these residues after blasting is the discoloration, visible to the naked eye, of the Al2O3 surface. In production, blasting is usually repeated or modified in order to remove residual TiCxNyOz, but this often results in damage, such as flaking of the coating at the cutting edge line. It is therefore important to find a solution to this problem, especially for thin Al2O3 coatings, where usually lower blasting pressures are used in order not to damage the coating at the cutting edge, thus being subject to a higher risk of having TiCxNyOz residues after the blasting process.
In U.S. Pat. No. 6,426,137, a titanium oxide layer is utilized in order to reduce smearing onto the cutting edge. In this case the titanium oxide layer is fully covering the Al2O3 surface, acting as the top layer with a thickness of 0.1-3 μm. In another embodiment the titanium oxide layer is coated with a TiN layer.